Field of the Invention
The present invention relates to the method of preventing premature inversion or destabilization of polyacrylamide water-in-oil emulsions while silicon polyether copolymer breaker surfactant or organic breaker surfactant or a combination of these is blended with the emulsion. Such emulsions of polyacrylamide polymers or copolymers are typically used as friction reducers in hydraulic fracturing applications.
Background of the Art
In the oil & gas industry, polyacrylamide water-in-oil emulsions are used as additives in fracturing (fracking) and drilling applications. In fracturing, a large volume of fracturing fluid is injected under high pressure into the formation in order to fracture the rock formation (shale, sandstone, limestone, etc.) and this way liberating crude oil and/or natural gas from the pores. In hydraulic fracturing applications, high molecular weight anionic and cationic polyacrylamides are used as friction reducers where the hydrated polymer suppresses the turbulence present in high velocity gradient water. Since hydraulic fracturing is performed at very high injection rates, reduction of pumping pressure as a result of friction reduction will result in less power consumption by the pumps and thus the use of effective friction reducers in fracturing fluids can decrease the cost of the operation by a large margin.
Water-in-oil emulsions of polymers of acrylamide monomers or copolymers of acrylamide and acrylate monomers are those in which the continuous phase is oil and the polymer is present in the aqueous emulsion drops (dispersed phase). Upon dilution with water-based fluids the emulsion inverts and releases the polymer into the fluid and the polymer chains hydrate. This process is known as inversion. An advantage of the water-in-oil emulsion form is that the polyacrylamide is partially hydrated in the aqueous drop phase and thus it is more easily hydrated further, without making the emulsion highly viscous.
The above water-in-oil polyacrylamide emulsions may also comprise “breaker surfactants”. These can be medium (7-12) to high (12-16) HLB value (Hydrophile Lipophile Balance) surfactants. The role of the breaker surfactants is to destabilize the emulsion and promote the phase inversion upon dilution so that the polyacrylamide can easily disperse in the water phase when the water-in-oil polyacrylamide emulsions are diluted with water or brine. Therefore, these surfactants are known as “breaker surfactants” or “invertors” for water-in-oil emulsions. Silicon surfactants of medium to high HLB values (˜8-16) can also destabilize the water-in-oil emulsion by phase inversion and hence can be used as breaker surfactants for water-in-oil polyacrylamide emulsions.
A potential problem with the use of breaker surfactants is that they can also enable premature inversion of the water-in-oil polyacrylamide emulsions prior to dilution during storage, handling etc. This causes a rapid increase in the viscosity of the emulsion as part of the dispersed phase is inverted into the continuous oil phase which leads to a significant increase in the emulsion's viscosity, and causes separation and handling problems. Such emulsion blends are considered as unstable and will not deliver the required performance upon dilution with water or brine. Therefore, a method is desired to prevent the inversion of the polyacrylamide water-in-oil emulsion while breaker surfactant(s) are present in the emulsion's continuous, oil phase. The present invention discloses a novel method to prevent the premature inversion of water-in-oil polyacrylamide emulsion when one or more breaker surfactant is mixed with the emulsion.